PTL 1 discloses an automatic adjustment method of a serial interface circuit configured to adjust an output characteristic of a transmitter transmitting a signal to a serial interface. This method aims to perform the automatic adjustment of the transmitter without violating the protocol of the serial interface, and to this end includes: sequentially changing an output characteristic value of the transmitter; sending response frames added with information explicitly indicating retransmission of a frame to a coupled device with the output characteristic value thus changed via the serial interface; receiving the reception results of each of the plurality of response frames from the coupled device via the serial interface; and determining an adjustment value of the output characteristic value of the transmitter from the reception results of each of the response frames.